Purine or pyrimidine derivatives have anti-cancer and antiviral activities, and more than 10 kinds of the compounds including AZT, 3TC and ACV have already been commercialized. Particularly, since acyclic nucleoside phosphonate derivatives show a potent antiviral effect, cidopovir has been commercialized as an antiviral agent and many compounds including PMEA and PMPA now entered into the step of clinical trials. However, the earlier developed compounds were not perfect in the aspects of toxicity or pharmaceutical activity, and thus, it is still desired to develop a compound having no toxicity as well as a superior activity. The prior researches for purine or pyrimidine derivatives or acyclic nucleoside phosphonate derivatives as reported heretofore are as follows. Patents: U.S. Pat. No. 5,817,647; U.S. Pat. No. 5,977,061; U.S. Pat. No. 5,886,179; U.S. Pat. No. 5,837,871; U.S. Pat. No. 6,069,249; WO 99/09031; WO96/09307; WO95/22330; U.S. Pat. No. 5,935,946; U.S. Pat. No. 5,877,166; U.S. Pat. No. 5,792,756; Journals: International Journal of Antimicrobial Agents 12 (1999), 81-95; Nature 323 (1986), 464; Heterocycles 31(1990), 1571; J. Med. Chem. 42 (1999), 2064; Pharmacology & Therapeutics 85 (2000), 251; Antiviral Chemistry & Chemotherapy 5 (1994), 57-63; Bioorganic & Medicinal Chemistry Letters 10 (2000) 2687-2690; Biochemical Pharmacology 60 (2000), 1907-1913; Antiviral Chemistry & Chemotherapy 8 (1997) 557-564; Antimicrobial Agent and Chemotherapy 42 (1999) 2885-2892.